The circadian clock plays a key role in coordinating many biological processes from behaviors to cellular metabolism and mitosis. Recent studies have linked disruption of circadian rhythms with increased breast cancer risk, suggesting women working alternative shifts may have a higher risk of breast cancer. Our recent results indicate that disruption of molecular timing negatively impacts several processes that determine the outcome of polyaromatic hydrocarbon (PAH) metabolism, resulting in increased DNA damage. We have found that time of day, as well as individual circadian clock components, uniquely regulate processes governing benzo-a-pyrene (BaP) activated aryl hydrocarbon receptor (AHR) and estrogen receptor (ER) mediated endocrine disruptor (EDC) signaling in the mouse mammary gland and breast cancer cell lines. This new knowledge led us to hypothesize that the circadian clock influences BaP-mediated metabolism and DNA damage and repair, and that time of day, or disruption of the circadian clock, impacts the outcome of PAH exposure and EDC activity. To test this hypothesis we propose two Specific Aims. In Aim 1, we will define the circadian windows of susceptibility in mouse mammary tissues and MCF7 breast cancer cells by analyzing differences in BaP-induced AHR- and ER- signaling, formation and persistence of BaP-DNA adducts and DNA damage responses and the time course of BaP clearance using a novel multiphoton imaging approach to measure real time circadian-dependent differences in BaP metabolism in vitro and in vivo. In Aim 2, we will utilize a novel mammary gland transplant approach to discern for the first time in vivo central versus peripheral affects mediating circadia clock influences on acute BaP-induced toxicity and estrogen action in mammary tissues using circadian and AHR and ERa knockout mouse models and binding of AHR and ER in vivo by ChIP-Seq analysis. The expected outcomes of this research will be identification of the key sites of action and metabolic pathways by which PAHs interact with circadian clocks in promoting DNA damage and breast cancer.